The goal of these studies is the definition of the mechanisms for the regulation of methionine metabolism in mammalian tissues. Previous studies indicate the existence two regulatory sites. The first is the distribution of available methionine between prtein synthesis and the formation of S-adenosylmethionine (AdoMet). The second, formed by the enzymes which utilize homocysteine (adenosylhomocysteinase, cystathionine synthase, betaine-homocysteine methyltransferase (BHMT), and methyltetrahydrofolat-homocysteicysteine methyltransferase), governs the distribution between methionine conservation and degradation by means of the transsulfuration pathway. An in vitro simulation of the latter regulatory site in liver indicates the significant contributions of both the tissue content of the enzymes and the concentrations of substrates and other effectors. For this reason, an important immediate goal is the identification of the factors which regulate the hepatic content of betaine. The methods employed will include assays of hepatic betaine, choline oxidase and BHMT together with a study of the dynamics of betaine metabolism in the isolated, perfused rat liver. Additional aims include the extension of the above concepts for metabolic regulation to extrahepatic tissues. Similarly, the hepatic mechanism exists in the cytosol, however important reactions occur at other subcellular sites. Thus the processes regulating the movement of metabolites between these sites will be studied. The clinical relevance of this area is extensive. Disorders of the methionine metabolism have been suggested in selected populations with arteriosclerotic cardiovascular disease; in patients with affective disorders; in oncogenesis; and in nutritional (alcoholic) liver diseases. In addition, S-adenosylmethionine has been employed as a therapeutic agent in patients with cholestatic syndromes, osteoarthritis and affective disorders.